Myelin is a vital component of the central and peripheral nervous system. Consisting of 70% lipid and 30% protein, myelin is formed both by oligodendrocytes (OLs) in the central nervous system (CNS) and by Schwann cells in the peripheral nervous system (PNS). Working as insulation, myelin enhances the speed and integrity of nerve signal propagation down the neural axon, allowing signals to pass back and forth between the brain and the nerves of the periphery over long distances. Myelin is also neuroprotective and maintains the long-term integrity of axons. Damage to the myelin sheath from any number of causes can lead to a variety of neurological disorders with often devastating consequences.
Myelination is a multistep process in which a myelinating cell adheres to an axon, then ensheaths and wraps it, culminating with exclusion of the cytoplasm from the spiraling processes to form compact myelin. The myelin sheath is formed by the plasma membrane, or plasmalemma, of glial cells: oligodendrocytes in the CNS, Schwann cells in the PNS. The plasmalemma makes contact with the axon and then begins to wrap around it, spiral fashion, the inner mesaxon continuing to circle the axon as the plasmalemma grows and flattens, squeezing out most of the cytoplasm, until the end result is a laminated sheath consisting of multiple concentric lamellae formed of plasma membrane, each lamella consisting of a total of four lipid leaflets.
The myelin sheath is formed in segments along the length of the axon. Between segments are small unmyelinated areas known as the nodes of Ranvier. This arrangement allows for very fast neural impulse transmission via saltatory conduction, in which the active components of impulse propagation are concentrated at the nodes of Ranvier, while current flow within the axon takes place in the internodes. The integrity of the nerve conduction process can be assessed clinically through measurements of conduction velocity. When myelination fails at a particular region of axon, the spread of the action potential slows down or stops altogether, measured clinically as slowed conduction or conduction block, respectively.
Despite the importance of myelin for the rapid conduction of action potentials, little is known about the mechanism of myelination.
Disorders of myelination can produce significant impairment in sensory, motor and other types of functioning when nerve signals reach their targets slowly, asynchronously, intermittently, or not at all. Disorders of myelination are also associated with progressive loss of the axons which further contributes to neurological impairment. Disorders of myelination can be demyelinating, as a result of removal or degradation of myelin already formed; or dysmyelinating, as a result of deficient or defective myelin development or maintenance. Many disorders affect both CNS and PNS myelin. Included among the more common disorders of CNS myelination are multiple sclerosis (MS), the leukodystrophies, the Guillain Bane Syndrome, and the Charcot Marie Tooth inherited peripheral neuropathies.
Multiple sclerosis (MS) is a progressive autoimmune inflammatory and demyelinating disease of the central nervous system (CNS). The pathological hallmarks of MS are white and grey matter demyelination, inflammation, axon damage, and blood-brain barrier (BBB) disruption (Dhib-Jalbut, Neurology, vol. 68, no. 22, supplement 3, pp. S13-S54, 2007; Holmoy and Hestvik, Current Opinion in Infectious Diseases, vol. 21, no. 3, pp. 271-278, 2008; Lisak, Neurology, vol. 68, no. 22, supplement 3, pp. S5-S12, 2007, discussion S43-S54). The etiology of MS is still not clear, but MS is classically characterized by proinflammatory T helper (Th) cells, Th1 and Th17 infiltration into the CNS (Dhib-Jalbut, Neurology, vol. 68, no. 22, supplement 3, pp. S13-S54, 2007; Fletcher et al., Clinical and Experimental Immunology, vol. 162, no. 1, pp. 1-11, 2010). Whether the disease manifests in repeated episodes of inflammation or as a chronic condition, it often results in multiple scars, or plaques in the brain, that contribute to the impairment or loss of nerve function. MS, while primarily affecting young adults, can manifest in patients of any age. Symptoms of MS include, for example, impaired vision or cognitive function, numbness, weakness in extremities, tremors or spasticity, heat intolerance, speech impairment, incontinence, or impaired proprioception. Patients with MS often also present with depression.
An important but elusive goal in MS research is to enhance remyelination, i.e., the generation of new myelin sheaths to insulate and protect axons that have lost their myelin in the central nervous system (CNS). Remyelination is normally quite limited in MS (Prineas et al., 1993), particularly in the later stages of the disease when neurodegeneration, including loss of axons dominates (Franklin and French-Constant, 2008). Remyelination is expected to have the dual benefit of restoring saltatory conduction (Smith et al., 1979) to demyelinated axons and preventing axonal loss, a major source of morbidity in this disease (Bruce et al., 2010; Dubois-Dalcq et al., 2005). While there has been progress in developing therapies to prevent the immune-mediated damage in MS, progress in remyelination and neuroprotective therapies has been much more limited (Mullard, 2011). There are no currently approved therapies to promote remyelination with only one, LINGO, in clinical trials (Mullard, 2011).
Insufficient myelination in the central nervous system has also been implicated in a wide array of other neurological disorders. Among these are forms of cerebral palsy in which a congenital deficit in forebrain myelination in children with periventricular leukomalacia, contributes to neurological morbidity (Goldman et al., 2008). At the other end of the age spectrum, myelin loss and ineffective repair may contribute to the decline in cognitive function associated with senescence (Kohama et al., 2011). Therefore, effective strategies to enhance remyelination may have substantial therapeutic benefits in halting disease progression and restoring function in MS and in a wide array of neurological disorders.
Remyelination requires expansion of precursor cells, their recruitment to demyelinated axons and their subsequent differentiation into oligodendrocytes (OLs), and the formation of myelin sheaths around demyelinated axons by these newly differentiated axons (Franklin and French-Constant, 2008; Zhao et al., 2005). The precise precursor cells responsible for remyelination and the reasons why they are ineffective in the repair of demyelinated lesions in multiple sclerosis and other disorders is not known. Recent evidence suggests that there are at least two sources of remyelinating cells in the adult human and mouse brain. One is the pool of oligodendrocyte progenitor cells (OPCs) present in the parenchyma of healthy brain as well as in, and around MS lesions (Scolding, N. et al. Brain 121 (Pt 12), 2221-2228, 1998; Picard-Riera et al., Proc. Natl. Acad. Sci. U.S.A., 99, 13211-13216, 2002; Menn et al., J. Neurosci., 26, 7907-7918, 2006). OPCs, which can be identified by their expression of the NG2 chondroitin sulfate proteoglycan and platelet-derived growth factor receptor alpha (PDGFRα), respond locally to demyelination by generating oligodendrocytes. They have limited self-renewal capacity and do not migrate long distances during remyelination which may contribute to their depletion around lesion sites (Gensert and Goldman, Neuron, 19:197-203, 1997; Franklin et al., J. Neurosci. Res., 50:337-344, 1997). Another source of remyelinating oligodendrocytes are the glial fibrillary acidic protein (GFAP)-expressing multipotent stem cells (type B cells) in the subventricular zone (SVZ) (Menn et al., J. Neurosci., 26, 7907-7918, 2006). These cells can self-renew and generate all neural cell types, i.e. neurons, astrocytes and oligodendrocytes, in response to a variety of morphogenic signals including the secreted morphogen Sonic Hedgehog (Shh) (Ahn and Joyner, Nature, 437:894-897, 2005).
While generation of oligodendrocytes from the adult SVZ is normally modest (Ahn and Joyner, Nature, 437:894-897, 2005), it significantly increases in response to demyelination, including in patients with MS (Nait-Oumesmar et al., Proc. Natl. Acad. Sci. U.S.A., 104:4694-4699, 2007). The signals that drive SVZ expansion in response to demyelination are not well established.
Shh is required for the generation of oligodendrocytes during development (Nery et al., Development, 128:527-540, 2001; Tekki-Kessaris, Development, 128:2545-2554, 2001) and for the maintenance of stem cells in the adult SVZ (Ihrie et al., Neuron, 71:250-262, 2011; Balordi and Fishell, J. Neurosci., 27:14248-14259, 2007). Increased Shh expression has been found to be present in active MS lesions (Wang et al., Ann Neurol., 64:417-427, 2008). Canonical Shh signaling is mediated by interactions of the hedgehog receptor patched (Ptc) with the G-protein coupled transmembrane co-receptor smoothened (Smo). Binding of Shh to Ptc relieves its inhibition of Smo and thereby activates the Gli family of zinc finger transcription factors (Ingham and McMahon, Genes Dev., 15:3059-3087, 2001; Ruiz i Altaba et al., (2002) Nat. Rev. Cancer 2, 361-372). The GLI zinc-finger transcription factors have been suggested to be essential for the mediation of HH signals (Ingham & McMahon, (2001) Genes Dev. 15, 3059-3087; Ruiz i Altaba et al., (2002) Nat. Rev. Cancer 2, 361-372; Ruiz i Altaba et al., (2004) Cancer Lett. 204, 145-157). GLIs participate in the final step of the Hh/GLI signaling pathway, and they regulate several genes, including those that are related to cell cycle control and Hh/GLI signaling (Eichberger et al., Genomics 2006, 87, 616-632). GLI1 acts as a transcriptional activator, whereas GLI2 and GLI3 act as both activators and repressors (Matise, and Wang (2011) Curr Top Dev Biol 97, 75-117; Aza-Blanc et al., Development 2000, 127, 4293-4301). All GLIs bind to DNA through five zinc-finger domains that recognize the consensus GLI-selective sequence 5′-GACCACCCA-3′ (SEQ ID NO: 1), which regulates transcription (Kinzler et al., Nature 1988, 332, 371-374; Kinzler and Vogelstein, Mol. Cell. Biol. 1990, 10, 634-642). Of the three Gli proteins, Gli1 expression is considered a sensitive readout for, and an indicator of the highest levels of Shh signaling (Ahn and Joyner, Cell, 118:505-516, 2004). However, Gli1 is apparently redundant in mouse development and tumorigenesis, and there is to date no data on the requirement for GLI1 in human cells (Park et al., (2000) Development (Cambridge, U.K.) 127, 1593-1605; Weiner et al., (2002) Cancer Res. 62, 6385-6389).